This invention relates to a method for improving the electrical characteristics of an electrical insulating hydrocarbon.
More specificially, the invention relates to a method for improving the electrical characteristics of an electrical insulating hydrocarbon. The improved electrical insulating hydrocarbon of the present invention is suitable for use in oil-filled or oil-impregnated electrical appliances in which insulating materials or dielectric materials made of plastics, such as polyolefin, are at least partly employed.
2. Description of the Prior Art
Electrical appliances such as oil-filled capacitors, oil-filled power cables and transformers have recently been made to withstand high electric voltages while being small in size. With this tendency, various kinds of plastic films are used together with conventional insulating paper.
There are several electrical insulating hydrocarbon oils known that are filled in or impregnated in these electrical applicances. However, the conventional electrical insulating oils such as refined mineral oils, polybutenes, alkylbenzenes, diarylalkanes, diarylcycloalkanes, arylindanes, alkylbiphenyls, cycloalkylbiphenyls, alkylnaphthalenes and cycloalkylnaphthalenes have several drawbacks. Further, conventional electrical insulating hydrocarbon oils are not completely compatible with the above-mentioned plastic materials, such as polyolefin films, which have recently come into use in oil-filled electrical appliances.
With the requirements of high-voltage durability and size reduction, it is necessary that the electrical insulating oil have a high dielectric breakdown voltage, a low dielectric loss tangent, and good hydrogen gas absorbing capacity. The hydrocarbon gas absorbing capacity indicates the stability of the insulating oil against corona discharge (partial discharge) under high electric voltage conditions. The higher the gas-absorbing capacity, the smaller the likelihood of corona discharge, which means that the insulating oil has excellent stability and/or durability.
In order to meet the requirement of high-voltage use, plastic films such as polyolefins films, polystyrene films and polyester films are used to either partially or completely replace the conventional insulating paper as insulating materials or dielectric materials for electrical appliances such as oil-filled electric cables and capacitors. In view of their dielectric strength, dielectric loss tangent and dielectric constant, polyolefin films especially polypropylene and cross-linked polyethylene films, are preferred insulating or dielectric materials.
When these polyolefin films are impregnated with insulating oils, some oils cause the films to swell to some extent. If a film becomes swollen, the thickness of the insulating layer increases. As a result, the resistance to the flow of insulating oil increases in electrical cables, and insufficient impregnation with insulating oil occurs in electric capacitors. This causes the formation of voids (un-impregnated portions) and the undesirable effect or lowering of the corona discharge voltage.
In connection with the above-mentioned conventional electrical insulating oils, the values of the dielectric breakdown voltages (BDV) and the dielectric loss tangents (tan.delta.) are satisfactory to a certain extent, but the hydrogen gas absorbing capacity or corona discharge characteristics and the stability of the dimensions of the polypropylene films are not satisfactory.
British Pat. No. 2,082,626 to Sato, et al. describes an electrical insulating oil that comprises aromatic hydrocarbons having two aromatic nuclei, such as diarylalkanes. The insulating oil is prepared by adding unsaturated dimers of styrene or its methyl homolog to diarylalkanes.
U.S. Pat. No. 4,111,824 to Schulz, et al. describes an electrical insulating oil prepared from the heavy oil fraction obtained as a by-product in the preparation of ethylbenzene. The insulating oil is recovered by distillation from a fraction which contains diarylalkanes.
In view of the state of the art, a need exists to provide an improved electrical insulating oil that overcomes the deficiencies of existing insulating oils.
It is an object of this invention to provide an electrical insulating substance that has an excellent dielectric constant, a good hydrogen gas absorbing capacity and a high compatibility with insulating or dielectric materials made of plastic.
It is another object of this invention to provide oil filled electrical appliances that have excellent corona discharge characteristics, dielectric breakdown voltage and maintain a long service life. The achievement of these and other objects will be apparent from the following description of the subject invention.